1. Field of the Invention
This invention relates generally to circuit breakers for transmission lines and more particularly to circuit breakers for transmission lines having parallel resistor interrupters for reducing an overvoltage condition which appears on the transmission lines when it is connected.
2. Description of the Prior Art
If an overvoltage appears on a transmission line or bus bar to which circuit breakers are attached, it is desirable to reduce the overvoltage which appears on the transmission line when it is connected. In order to reduce the overvoltage, prior art circuit breakers have utilized a plurality of main interrupters connected in series between end terminals with parallel resistor interupters connected through parallel resistors in series between the end terminals and in parallel with the main interrupters. The connections between the interrupters are changed so as to change the value of the resistance in parallel with the main interrupters. In prior art devices, the connections are changed at least twice so as to present at least two values of resistance before the main interrupters are closed.
Various mechanical mechanisms have been used in the prior art to chronologically change the connections between the interrupters so as to change the resistance. Mechanisms of this sort are shown in FIGS. 1 and 2. FIG. 1 shows a circuit diagram of a monopole multi-gap circuit breaker according to the prior art. As shown in FIG. 1, a multi-gap circuit breaker has 2 main interrupters 1 connected in series between end terminals 8 and 9. Parallel resistor interrupters 2, 3 are connected respectively in parallel with each corresponding main interrupter 1. Parallel resistors 6a and 6b are connected in series with one another and the parallel resistor interrupters 2 between the end terminals 8 and 9. The parallel resistor interrupters 3 are connected from a point between parallel resistors 6a and 6b to each end terminal 8 and 9. The point between the parallel resistor interrupters 2 and the point between the main circuit interrupters 1 are connected electrically with each other. In operation, the main interrupters 1 and parallel resistor interrupters 2, 3 are mechanically connected to a driving device 7 through a rod 4 and insulating rods 5 and 10. Interrupters 3 are mechanically but not electrically interconnected. Insulating rods 5 are used for closing and opening the parallel resistor interrupters 3 since the interrupter 3 should be insulated from the rod 4 which is connected mechanically and electrically to the main circuit interrupters 1 and parallel resistor interrupters 2.
During the operation of the circuit breaker, the main interrupter 1 and parallel resistor interrupters 2, 3 are so arranged that the interrupters 1, 2, 3 will close at slightly different moments of time. However, the interrupters which have the same reference numerals will close at the same time. When the circuit breaker is closing, at a first stage the parallel resistor interrupters 2 are closed by the insulating rod 10 which is operated by the driving device 7. The parallel resistors 6a, 6b are then connected directly to the end terminals 8, 9. The ohmic value between the end terminals 8 and 9 amounts to 2(Ra+Rb), wherein Ra is the ohmic value of the parallel resistor 6a and Rb is the ohmic value of the parallel resistor 6b. At a second stage, the parallel resistor interupters 3 are closed by the rod 4, insulating rods 5 and insulating rod 10 which are connected mechanically with each other. The parallel resistors 6a are connected directly to the end terminals 8 and 9. The ohmic value between the end terminals 8, 9 amounts to 2Ra. At the final stage the main interrupters 1 are closed by the rod 4 and the insulating rod 10. The construction of the circuit-breaker shown in FIG. 1 needs 2 parallel resistor interrupters 2, 3 and the insulating rods 5 which operate the parallel resistor interrupters 3. The mechanism which makes the interrupters 1, 2, 3 close at slightly different moments of time has low reliability, due to the complicated nature of the connecting mechanism between the movable contacts of the interrupters 1, 2 and 3.
FIG. 2 shows a circuit diagram of a monopole multi-gap circuit breaker according to the prior art. As shown in FIG. 2, a multi-gap circuit breaker has 2 main interrupters 1 connected in series between the end terminals 8 and 9. Parallel resistor interrupters 2 and 3 are connected respectively in parallel with each corresponding main interrupter 1 between the end terminals 8 and 9. The parallel resistors 11 are inserted between the parallel resistor interrupters 2. The parallel resistors 12 are inserted between the parallel resistor interrupters 3. The point between main breakers 1 and the points between the parallel resistors 11, 12 are connected electrically. The distance L.sub.1 between the contacts of the parallel resistor interrupters 2 is smaller than the distance L.sub.2 between the contacts of the parallel resistor interrupters 3. The ohmic value of parallel resistor 11 is greater than that of parallel resistor 12. The interrupters 1, 2, 3 are so arranged that they will close at slightly different moments of time. When the circuit breaker closes, at the first stage, the parallel resistor interrupters 2 are closed; at the second stage, the parallel resistor interrupters 3 are closed; and at the final stage, the main interrupters 1 are closed. The resistance at the first stage is therefore greater than at the second stage. The closing time lag between the parallel resistor interrupters 2 and 3 depends on the difference between L.sub.1 and L.sub.2. The construction of the circuit breaker shown in FIG. 2 requires parallel resistors 11 and 12 which have different ohmic values and parallel resistor interrupters 2 and 3 for each main interrupter 1. It is inevitable that the driving devices 7 must drive the interrupters 1, 2, 3 and therefore have low reliability.